Conventionally, the following radiographic system is known:
After radioactive rays have been transmitted through a photographic object, they are absorbed by a fluorescent substance, so that the radioactive ray image information is recorded in the fluorescent substance. Then the fluorescent substance is scanned and excited with laser rays, and light is emitted from the fluorescent substance. This emitted light is detected by a photo detector. Light beams are modulated by this detected radioactive ray image information, and the radioactive ray image is recorded on a recording medium such as a photographic film.
Compared with a conventional silver salt type radioactive ray photographic system, the above radioactive ray photographic system in which the fluorescent substance is used has a wide latitude of radioactive ray exposure so as to record an image. From this point of view, the above radioactive ray photographic system has a very high utility value. The above radioactive ray photographic system is effectively applied to an X-ray photographic system for photographing human bodies.
X-rays are harmful to human bodies when the exposure dosage is increased. Therefore, it is desirable to make an amount of information provided by one photographing operation as large as possible. However, X-ray photographic films to be used at present must have not only an aptitude for photographing but also an aptitude for observation. Therefore, the X-ray photographic films to be used at present have both aptitudes to some extent. For this reason, concerning the aptitude for photographing, the X-ray exposure latitude is not sufficiently wide, and concerning the aptitude for observation, the image quality is not sufficiently high for medical diagnosis.
In order to solve the above problems, the following method for processing radioactive ray images are disclosed:
(1) Japanese Patent Publication No. 62373/1987 (Conventional Example 1) PA1 (2) Japanese Patent Publication No. 62383/1987 (Conventional Example 2) PA1 X=Sorg-Sus. PA1 F(X) is described below. PA1 When .vertline.X1.vertline.&lt;.vertline.X2.vertline., the inequalities F'(X1).gtoreq.F' and (X2).gtoreq.0 are satisfied. With respect to X0 that satisfies .vertline.X1.vertline.&lt;.vertline.X0.vertline.&lt;.vertline.X2.vertline., F(X) is a monotone increasing function that satisfies the inequality F'(X1) &gt;F'(X2). PA1 F(Sorg)=(1+.beta.(Sorg)).multidot.Sorg PA1 G(Sorg, Sus)=-.beta.(Sorg).multidot.Sus PA1 P(Sorg, Sus)=(1+.beta.(Sus)).multidot.Sorg PA1 Q(Sus)=-.beta.(Sus).multidot.Sus PA1 A(Sorg): Function of Sorg excluding Sus PA1 B(Sus): Function of Sus excluding Sorg
This method is described below:
A fluorescent substance is scanned with rays of exciting light, so that radioactive ray image information recorded in the fluorescent substance is read out. After the image information has been converted into electric signals, it is reproduced on a recording medium in the following manner. An unsharp mask signal Sus corresponding to an extremely low frequency is found at each scanning point. Operation is carried out in accordance with the following expression. EQU S'=Sorg+.beta.(Sorg-Sus) (1)
where Sorg is an original image signal that has been read from the fluorescent substance, .beta. is an emphasis coefficient, and S' is a reproduction image signal. In this way, frequency components not lower than the above extremely low frequency are emphasized.
In the above expression, the unsharp mask signal Sus corresponding to the extremely low frequency is defined as a signal in which the original image is blurred so that the original image only contains low frequency components lower than the extremely low frequency component. In this method, the emphasis coefficient .beta. is simply increased in accordance with an increase of the original image signal Sorg or the unsharp mask signal Sus. Further, in this method, the unsharp mask signal Sus is found when the original image signals Sorg are simply averaged in the mask.
According to this method, the frequency components not lower than the extremely low frequency which are effective for medical diagnosis are emphasized so that the contrast can be enhanced. In this way, the medical diagnosis performance can be improved.
This method is described below:
An accumulation type fluorescent substance is scanned with rays, so that radioactive ray image information recorded in the fluorescent substance is read out. After the image information has been converted into electric signals, it is reproduced as a visual image on a recording medium in the following manner. An unsharp mask signal Sus corresponding to an extremely low frequency is found at each scanning point. Operation is carried out in accordance with the following expression. EQU S'=Sorg+F(X) (2)
(In this case, X and F(X) are expressed as follows.
where Sorg is an original image signal that has been read from the fluorescent substance, and S' is a reproduction image signal. In this way, frequency components not lower than the above extremely low frequency are emphasized.
In this case, values of F(X) can be stored in the form of a table.
There is a tendency that an artifact is generated in a region where the difference signal .vertline.Sorg-Sus.vertline. is high. From the viewpoint described above, according to this method, the frequency is more emphasized in a portion where the difference signal .vertline.Sorg-Sus.vertline. is high. Accordingly, it is possible to carry out image processing in which the generation of an artifact is suppressed.
According to the former conventional method described above, it requires a long period of time to carry out an operation. In order to reduce the operation time, it is suggested to employ a table-looking system in which the operation values are previously stored in the form of a table. However, when a ratio of emphasis of frequency, which is the emphasis coefficient .beta. in the conventional example 1, is changed in accordance with an increase of Sorg or Sus, the following problems may be encountered.
When the emphasis coefficient .beta. is a function which changes in accordance with a change in Sorg (in this case, the function is expressed by .beta. (Sorg)), the expression of the conventional example 1 can be transformed as follows. ##EQU1##
In this case,
(F is a function of Sorg, and G is a function of Sorg and Sus.)
When the emphasis coefficient .beta. is a function which changes in accordance with a change in Sus (in this case, the function is expressed by .beta.(Sus)), the expression of the conventional example 1 can be transformed as follows. ##EQU2##
In this case,
(F is a function of Sorg and Sus, and G is a function of Sus.)
In any cases, terms of both Sorg and Sus are included. Accordingly, when the values are stored in the table-looking system, it is necessary to provide a two-dimensional arrangement. Therefore, a large capacity of memory is required for storing the table.
In some cases, it is preferable that the table is calculated or transformed for each image. In this case, however, it takes a long period of time for table calculation. Therefore, it is not possible to reduce the operation time. Further, when .beta. is sharply changed, an artifact is generated, which may cause a wrong diagnosis. Further, when Sus is found by calculating a simple average of the signals in the mask, it is necessary to conduct a division, so that it takes a long period of time for operation.
In the latter conventional system described above, a table is required, the dimensions of which are (a range of values of Sorg+a range of values of Sus). Not only the addition and subtraction of each term but also the operation of the argument X (=Sorg-Sus) is required for each pixel. Therefore, it is difficult to shorten the operation time sufficiently. In order to omit the calculation of the argument X, X may be a function of Sorg and Sus like f (Sorg, Sus). In this case, however, a large capacity of table memory is required. Further, there is a description that f(X) may include a function of Sorg and/or Sus. In this case, a large capacity of table memory is also required in the same manner as that of the conventional technique (1) described before. In the description, there is provided no concept that the operation speed is increased when the function of only one of Sorg and Sus is added or subtracted.